1. Field of the Invention
This invention pertains to thermal management of fast charging high capacity batteries, and to associated cooling structures and methods for maintaining optimal efficiency in fast charging and subsequent use of such batteries.
2. Description of the Related Art
“Fast charging” refers herein to charging a battery at a rate of greater than 30 amperes per 100 ampere-hours of battery capacity. A goal of fast charging is to bring the state of charge (“SOC”) of a battery from 30% to 80% of full charge in less than about 1.5 hours.
Battery-powered vehicles such as forklifts, electric carts and the likes are widely used in industry and require periodic recharging of batteries to maintain vehicle operability.
U.S. patent application Ser. No. 10/287,753 filed Nov. 4, 2002 in the names of John F. Aker and James R. Wade for “FAST CHARGER FOR HIGH CAPACITY BATTERIES,” issued as U.S. Pat. No. 6,803,746 on Oct. 12, 2004, describes high-efficiency fast chargers for fast charging of high-capacity batteries. The disclosure of such prior co-pending U.S. patent application Ser. No. 10/287,753 is hereby incorporated herein by reference, for all purposes. “High capacity batteries” as used herein refers to batteries of greater than about 100 Amp-hours capacity.
As disclosed in the background section of our prior co-pending patent application, conventional battery chargers typically operate at charging rates that are about or below 20 amperes per 100 ampere-hours of battery capacity. In consequence, the charging rate of such conventional battery chargers decreases early in the charge cycle, and 8-12 hours are required to restore the battery to full charge status.
The fast charger apparatus and method disclosed in such prior co-pending application overcomes the deficiencies of conventional charging approaches, and permits lead-acid batteries commonly used to power forklifts, and other electric vehicles, to be safely charged at higher currents, in a highly effective manner, bringing batteries from 30% to 80% state of charge in about an hour. The charging enabled by the fast charging system of such prior co-pending application is on the order of 400% faster than conventional charging technologies.
By such high-rate, high-efficiency charging afforded by the fast charging system described in our prior co-pending application, it is possible to operate fleets of battery-powered vehicles without changing out batteries, thereby obviating the time, effort and lost productivity costs of such change-outs. The fast charging system of our prior co-pending application thereby permits a substantial increase in fleet productivity for such electric battery-powered vehicles. As a result, a typical operation requires at least 50% fewer batteries, in addition to the power savings realized by such systems.
In operation, the fast charging systems of our prior copending application entail unique thermal management issues in the high rate, high-efficiency regime involved in the charging operation. Specifically, the high-rate intensive charging enabled by the apparatus and method of our prior co-pending application along with constant use in a heavy two or three shift operation without time for the traditional “cool down” period, produces substantial increases in battery temperature during the charging operation. Such temperature increases in turn require charging current to be reduced in order to maintain battery temperature at a safe level to achieve desired battery life.
This occurrence of substantial increases in battery temperature during fast charging and heavy use applications undesirably impacts the fast charging operation by increasing the time required for a battery to achieve the targeted state of charge. In addition, higher battery temperatures result in reduced battery life and possible warranty issues with the battery. As a result, the potential benefits inherent in fast charging have not yet been fully realized. This is the problem to which the present invention is directed.